The present invention relates to a method for determining geometric imaging properties of a flat panel detector in an x-ray inspection system for non-destructive material testing, including the steps arranging a calibration phantom between an x-ray source and the flat panel detector, the calibration phantom comprising at least one discrete geometric object; recording at least one x-ray image of the calibration phantom with the flat panel detector, with at least one discrete geometric shape being recorded by imaging the at least one discrete geometric object of the calibration phantom; and determining a location-dependent distortion error of the flat panel detector from the at least one x-ray image on the basis of at least one characteristic of the at least one discrete geometric shape. An embodiment of the present invention further relates to a correspondingly adapted x-ray inspection system and a corresponding calibration phantom.
The testing accuracy of an x-ray inspection system for non-destructive material testing with a flat panel detector inter alia depends on the geometric model of the detector taken as the basis in reconstruction and evaluation matching the dimensions of the real detector as exactly as possible. Contrary to former assumptions it has turned out that flat panel detectors within the bounds of the desired high testing accuracies are not flat but show a bend or curvature of the detector surface. It is also possible that for example the pixel size of flat panel detectors is not constant but is a function of the location, i.e. depends on the line number and column number. Distortion errors are thus caused, i.e. the coordinate of the real image of a characteristic, for example of one point, compared to the ideal image is dislocated due to the curvature or the non-constant pixel size.
For the correction of distortion errors of a flat panel detector, document WO 2012 062543 A2 proposes a method for operating a measurement arrangement for a computer tomograph, with a calibration phantom being arranged between the radiation source and the flat panel detector, and at least one x-ray image of the calibration phantom being recorded with the flat panel detector, and a distortion error of the flat panel detector being determined from known dimensions of the calibration phantom and from the at least one x-ray image as a function of the location. The calibration phantom comprises a plurality of separate structures, for example spheres, the dimensions of which, i.e. size and distances, must be known exactly. This usually requires a high-precision measurement of the calibration phantom, for example by use of a coordinate measuring instrument, thus involving a lot of time and high costs.